M'OLLUSCA. 495 



complete it must have had much the form and proportions of 

 L. pelagica, with a height of about 30 mm. It differs, however, 

 from all other members of the genus in the New Jersey faunas, 

 in the smaller number of radiating ribs, there being only 12 

 or 13 in all, two or three oi these upon the posterior slope being 

 very faint; the interspaces between the ribs are much broader 

 than the ribs themselves. 



Formation and locality. — Navesink. marl, Mullica Hill (169). 



Geographic distribution. — New Jersey. 



Genus Pi,agiostoma Sowerby. 



Plagiostoma erecta (Whitfield). 



Plate LIV, Fig. 10. 



1886. Dosinia ? erecta Whitf., Pal. N. J., vol. i (Monog. U. 

 S. G. S., vol. 9), p. 162, pi. 18, fig. 17 ( ?i8-2o). 



Description. — Shell very thin, depressed convex, subcircular 

 in outline, a little longer than high, in large examples attaining 

 a length oi 25 mm., a height of 22.5 mm. and convexity of 4.5 

 mm. Beaks nearly erect, situated back of the middle of the 

 valves, antero-cardinal margin deeply inflected to form an exca- 

 vated escutcheon, back oi the beaks the shell is compressed 

 and produced into' a small triangular auriculation ; the anterior 

 extremity of the hinge-line meeting the anterior shell margin 

 in a slightly obtuse angle. The posterior margin slightly sinu- 

 ate above from the hinge extremity to' below the base of the 

 auriculation; beyond this point the margin describes a nearly 

 circular curve to the posterior hinge extremity. The surface 

 smooth and shining, marked by fine concentric lines of growth. 



Remarks. — 'This species was established by Whitfield "en- 

 tirely from the external appearances of a few shells destitute 

 O'f the most of the cardinal portions, and one internal cast, on 

 which the markings are so faint as not to be considered reli- 

 able," and was referred to the genus Dosinia. The internal cast 

 mentioned by Whitfield may belong to the same species as the 

 specimens preserving the shells, but it is not possible to affirm 



